is not a factor of $x^3-x$.

The correct option is D $x^2$
$x^3-x$ can be written as 

$x^3-x$ = $x(x^2-1)$
$x^3-x$ = $x(x-1)(x+1)$
So, x, x-1 and x+1 are the factors of  $x^3-x$

Mathematics